How much wood expands and shrinks with humidity: equilibrium moisture content, the moisture content to build at, and per-species movement coefficients for flatsawn and quartersawn stock.
Why wood moves
Wood only swells and shrinks below about 30 percent moisture content, and it moves across the grain, not along it. A board gets wider and narrower with the seasons; it stays essentially the same length.
Green wood holds water in two places: free water sitting in the hollow cell cavities, and bound water held inside the cell walls themselves. Drying removes the free water first, and losing it changes nothing dimensionally. Only when the cavities are empty and the walls begin to give up their bound water does the wood start to shrink. That crossover is the fiber saturation point, and it averages around 30 percent moisture content, give or take a few points by species and even between boards. Everything a woodworker calls wood movement happens below it. The movement is also strongly directional. Across the growth rings and around them, a board changes width noticeably. Along the grain it barely moves at all, typically one tenth to two tenths of one percent all the way from green to ovendry, which is why nobody designs for length. The exception is worth knowing: reaction wood and juvenile wood, often from near the center of the tree, can shrink up to 2 percent lengthwise, and that is where mysteriously bowed and twisted stock usually comes from.
Wood Handbook FPL-GTR-282, Chapter 4, Moisture Content and Shrinkage
Equilibrium moisture content
Whatever the surrounding air dictates. At 70 degrees F, wood settles near 6 percent moisture content in 30 percent humidity and near 13 percent in 70 percent humidity, so an ordinary heated shop swings its wood through roughly 7 points across a year.
Wood is hygroscopic, which means it trades moisture with the air until the two are in balance. The balance point is the equilibrium moisture content, and it is governed overwhelmingly by relative humidity, with temperature playing only a small part. This is the number that matters, because a board does not care what moisture content it was sold at. It cares where the air is taking it. The table below covers the conditions most shops and houses actually see: a cold garage at 50 degrees F, a heated shop at 70, and a hot attic or summer porch at 90. Read down to your humidity and across to your temperature. Notice how little the temperature columns differ and how much the humidity rows do. If you want a value for conditions not shown, the Handbook publishes a sorption equation that covers the whole range continuously, and the figures here are computed from it, which is why they match the published table to within a tenth of a percent.
| Relative humidity (%) | At 50 deg F (% MC) | At 70 deg F (% MC) | At 90 deg F (% MC) |
|---|---|---|---|
| 30 | 6.3 | 6.2 | 5.9 |
| 40 | 7.9 | 7.7 | 7.4 |
| 50 | 9.5 | 9.2 | 8.9 |
| 60 | 11.2 | 11.0 | 10.5 |
| 70 | 13.4 | 13.1 | 12.6 |
| 80 | 16.4 | 16.0 | 15.4 |
| 90 | 20.9 | 20.5 | 19.8 |
Wood Handbook FPL-GTR-282, Chapter 4, Table 4-2 and Eq. 4-5
Moisture content to build at
Build interior work at about 8 percent moisture content in most of the United States, about 6 percent in the dry southwest, and about 11 percent on a damp warm coast. Exterior work goes together nearer 12 percent.
The goal is not to build at some universally correct moisture content. It is to build at the moisture content the piece will live at, so that whatever movement is coming has already happened before you cut a joint. A cabinet assembled from wood at 12 percent and then installed in a house that runs at 8 will shrink in place, and the panels, joints and finish will show it. Wood installed too dry does the reverse and can crush its own fibers as it swells against a tight fit. The regions below are broad climate zones, not state lines, and the average column is the target while the individual pieces column is the acceptable spread within a batch. Two practical notes. Stock delivered from a kiln is not necessarily at your number, and it will drift toward your shop within weeks, so sticker it and let it settle rather than trusting the invoice. And a shop that is unheated in winter is not the same environment as the heated house the furniture is going to, which is the single most common reason well-built work opens up after delivery.
| Use | Region | Target average (% MC) | Individual pieces, low (% MC) | Individual pieces, high (% MC) |
|---|---|---|---|---|
| Interior: woodwork, flooring, furniture, trim | Most of the United States | 8 | 6 | 10 |
| Interior: woodwork, flooring, furniture, trim | Dry southwestern area | 6 | 4 | 9 |
| Interior: woodwork, flooring, furniture, trim | Damp, warm coastal area | 11 | 8 | 13 |
| Exterior: siding, trim, sheathing, laminated timbers | Most of the United States | 12 | 9 | 14 |
| Exterior: siding, trim, sheathing, laminated timbers | Dry southwestern area | 9 | 7 | 12 |
| Exterior: siding, trim, sheathing, laminated timbers | Damp, warm coastal area | 12 | 9 | 14 |
Wood Handbook FPL-GTR-282, Chapter 13, Table 13-2
Movement by species
Between roughly 0.002 and 0.004 inch per inch of width for every 1 percent change in moisture content, depending on species and on whether the board is flatsawn or quartersawn. American beech moves about twice what eastern white pine does.
These are the numbers to multiply by. Each coefficient gives the fractional change in a dimension for each 1 percent change in moisture content, so a coefficient of 0.00376 means a 1 inch width changes by 0.00376 inch per point, and a 36 inch width changes by 36 times that. Use the tangential coefficient for a flatsawn board, where the growth rings run roughly parallel to the face, and the radial coefficient for a quartersawn one, where they run roughly perpendicular. Most real boards are somewhere in between, and using the tangential figure is the conservative choice. The coefficients are valid between 6 and 14 percent moisture content and are referenced to the board's dimension at 10 percent, which covers essentially all indoor furniture and cabinetry. Outside that band the relationship stops being a straight line and the Handbook gives a different equation. Two rows need reading carefully. Hickory appears once because the source groups shagbark and pignut into a single true hickory figure. Western redcedar is measured from a 22 percent starting point rather than 30, which is noted in its own row.
| Species | Wood Handbook row | C_R - radial, quartersawn (per 1% MC) | C_T - tangential, flatsawn (per 1% MC) | Shrinkage starts at (% MC) |
|---|---|---|---|---|
| Red oak (northern red) | Red oak, northern red | 0.00137 | 0.00304 | 30 |
| White oak | White oak, white | 0.00194 | 0.00376 | 30 |
| Hard (sugar) maple | Maple, sugar | 0.00165 | 0.00353 | 30 |
| Soft (red) maple | Maple, red | 0.00137 | 0.00289 | 30 |
| Black cherry | Cherry, black | 0.00126 | 0.00248 | 30 |
| Black walnut | Walnut, black | 0.00190 | 0.00274 | 30 |
| White ash | Ash, white | 0.00169 | 0.00274 | 30 |
| Hickory (shagbark, pignut) | Hickory, true | 0.00259 | 0.00411 | 30 |
| Pecan hickory | Hickory, pecan | 0.00169 | 0.00315 | 30 |
| Yellow poplar | Poplar, yellow | 0.00158 | 0.00289 | 30 |
| Yellow birch | Birch, yellow | 0.00256 | 0.00338 | 30 |
| American beech | Beech, American | 0.00190 | 0.00431 | 30 |
| Douglas-fir (coast) | Douglas-fir, Coast-type | 0.00165 | 0.00267 | 30 |
| Southern yellow pine (loblolly) | Pine, loblolly | 0.00165 | 0.00259 | 30 |
| Eastern white pine | Pine, eastern white | 0.00071 | 0.00212 | 30 |
| Western redcedar | Cedar, western red | 0.00111 | 0.00234 | 22 |
Wood Handbook FPL-GTR-282, Chapter 13, Table 13-5
Quartersawn versus flatsawn
Yes, measurably. Across these species a flatsawn board moves between 1.3 and 3.0 times what a quartersawn board of the same species does, with a typical figure near 1.9 -- roughly half the movement for the same wood.
Wood shrinks more around the growth rings than across them, and that single fact is what makes quartersawn stock worth its price. The ratio between the two directions tells you how much you gain by quartersawing a given species. Yellow birch sits at the bottom near 1.3, where the sawing choice barely matters. Eastern white pine sits at the top near 3.0, where flatsawn moves three times what quartersawn does. There is a trap in reading these ratios, though, and it is worth stating plainly. A high ratio does not mean the wood moves a lot. Eastern white pine has the highest ratio in this set and also the least actual movement of any species here, because both of its coefficients are small to begin with. American beech has a middling ratio and the most movement, because its tangential coefficient is the largest. If you are choosing a species for stability, compare the tangential coefficients. If you have already chosen the species and are deciding how to have it sawn, compare the ratios.
Wood Handbook FPL-GTR-282, Chapter 13, Table 13-5
Estimating movement
Multiply the width by the species coefficient by the change in moisture content. A 36 inch flatsawn white oak top moving from 6 to 12 percent moisture content changes width by about 0.81 inch, which is why tops are never glued down at both edges.
The arithmetic is deliberately simple: change equals width times coefficient times the change in moisture content, in percentage points. The work is in choosing honest inputs. Take the width across the grain, not the length. Take the tangential coefficient unless you know the boards are genuinely quartersawn. And take a realistic moisture content swing rather than a comfortable one, which for a heated shop and house is commonly around 6 or 7 points between a dry winter and a humid summer. The worked example below uses a 36 inch flatsawn white oak top over a 6 point swing, and the answer is more than three quarters of an inch. That number is the reason for breadboard ends with elongated holes, for tabletop fasteners that slide in a slot, for panels that float in their frames, and for not gluing a wide top rigidly to an apron. The equation holds between 6 and 14 percent moisture content. Outside that range, or starting from green, the relationship curves and the Handbook gives a separate equation for it.
Wood Handbook FPL-GTR-282, Chapter 13, Eq. 13-3
Moisture meter accuracy
Within about 1 percent of the true average when used correctly on wood below fiber saturation. Pinless meters read from about 4 percent and pin meters from about 6, and both stop being quantitative above 30 percent.
A moisture meter is accurate enough to build by, provided you respect what it can and cannot see. Used properly on a quantity of lumber that has reached a steady moisture content below fiber saturation, corrected readings land within about 1 percent of the true average. That is a useful instrument. The limits are real, though. Pinless dielectric meters are reliable from about 4 percent up to about 30, and resistance meters with pins from about 6 to about 30. At either end of those ranges precision falls off, and any reading above roughly 30 percent should be treated as a yes-it-is-wet indication rather than a number. Two habits matter more than the meter you buy. Set the species correction, because density strongly affects the reading and an uncorrected figure on dense oak or hickory can be off by several points. And take several readings across a batch rather than one, since the meter reports the spot it touched and a stack that averages 8 percent can easily contain a board at 12.
Wood Handbook FPL-GTR-282, Chapter 13, Moisture Content Measurement